1. Field of the Invention
The present invention relates to a coated paper having high gloss and high bulk, and to a method of making said paper by coating and drum finishing the paper.
2. DESCRIPTION OF THE PRIOR ART
A method of finishing aqueous mineral coatings on paper is disclosed by Hart in U.S. Pat. No. 2,919,205 granted on Dec. 29, 1959 wherein the wet, coated web is pressed by an impervious backing means against a smooth heated surface having a temperature substantially above 100.degree. C, the external pressure applied being greater than the pressure of steam at the temperature of said heated surface. The aqueous coating at the time of its contact with the hot roll is sufficiently coherent so that it is not disrupted by the evolution of steam when the heated web is exposed to atmospheric pressure upon leaving the pressure nip.
One application of the basic teachings of Hart is known in the paper-coating art as "gloss-calendering" which, for the purpose of the present application, involves the production of a glossy surface on paper or related web materials by contacting the surface of a coated substrate with a polished finishing surface under temperature conditions sufficient to cause a temporary condition of plasticity in the surface thereof and under pressure conditions sufficient to smooth, mold and polish the surface to thereby obtain a high degree of finish or gloss without unduly compacting the substrate. This higher bulk leads to increased brightness and opacity, which are desirable properties in coated printing papers.
Suitable apparatus for gloss-calendering coated paper and related materials is described in, for example, U.S. Pat. No. 3,124,480 granted Mar. 10, 1964 to Mahoney et al for "Hot Pressure Finishing Apparatus for Web Materials."
Throughout the history of the prior art, efforts have been made to employ gloss-calendering techniques to produce a finished paper of high gloss. The advantage of gloss-calendering over other finishing techniques, particularly supercalendering, is that gloss-calendering results in minimal compacting of the base sheet, while supercalendering decreases bulk and densifies the base sheet. Supercalendering is generally destructive to many properties of the paper and in particular decreases brightness, opacity and bulk.
While gloss-calendering enjoys some use at the present time, it has not been successfully employed on a commercial scale to produce glossy finishes on paper. The primary reason for this failure is believed to be the competing and relatively incompatible requirements that the paper coating be wet enough to be molded by the gloss-calendering roll and at the same time be dry enough that the coating does not stick to the gloss-calendering drum. If the coating adheres to the gloss-calendering drum, it will be stripped away from the paper substrate as the paper is removed from the drum at the nip, thereby ruining the product.
A method by which high-bulk coated paper is purportedly produced is disclosed by Hain in U.S. Pat. No. 3,268,354 granted in 1966 and in U.S. Pats. No. 3,338,735 and 3,338,736 granted in 1967. In accordance with the invention of Hain, there is provided coated paper comprising a fibrous cellulosic substrate having bonded thereto a film comprising a major weight proportion of finely divided solid filler and a minor weight proportion of thermoplastic resin. In accordance with the technique of Hain, the coating is applied to the substrate and dried by heating to form an exposed coating surface which is dry to the touch. This step of drying the coating before gloss-calendering is in accordance with the teachings of the prior art as represented, for example, by Rice in U.S. Pats. Nos. 3,028,258 granted 1962 and 3,281,267 granted 1966. The resultant coated substrate is then passed through a nip between a pair of turning rolls, one of which is provided with a heated finishing surface for contacting the exposed surface of the coating. Together the rolls apply sufficient heat and pressure to cause the resin particles to coalesce at least partially.
These prior art techniques have not met with commercial success for reasons which will become apparent hereinafter.